A known desulfurization unit for LPG (corresponding to a source gas) is disclosed in JP2006-111766A (which will be hereinafter referred to as Reference 1). The desulfurization unit of Reference 1 includes first and second desulfurizers separately set at normal and high temperatures (the high temperature is higher than 100° C.). A desulfurizing agent for the first desulfurizer is utilized at normal temperature to adsorb odorous sulfur compounds such as tertiary-butylmercaptan (TBM), dimethyl sulfide (DMS), and the like that are included in a city gas (for example, a city gas 13A). The odorous sulfur compounds are relatively easily removed from the city gas by the desulfurizing agent of the first desulfurizer used at normal temperature. A desulfurizing agent for the second desulfurizer adsorbs sulfur compounds specifically included in LPG, by use of metal oxide such as nickel (Ni). The sulfur compounds included in LPG are, for example, carbonyl sulfide (COS) and the like that are relatively easily adsorbed by the desulfurizing agent. An operating temperature of the second desulfurizer depends on a desulfurization effect of the desulfurizing agent of the second desulfurizer.
JP2006-265480A (which will be hereinafter referred to as Reference 2) discloses a desulfurization unit basically including the same configuration as that of Reference 1. In particular, the desulfurization unit of Reference 2 includes a first desulfurizer operating at normal temperature and a second desulfurizer operating at a high temperature of 50° C. or higher. A desulfurizing agent accommodated in the second desulfurizer is used at the high temperature of 50° C. or higher. The desulfurizing agent of the second desulfurizer is provided for LPG. JPH5-114414A (which will be hereinafter referred to as Reference 3) discloses a first desulfurizer used at normal temperature and a second desulfurizer used at high temperature. The second desulfurizer serves as a hydrogenation desulfurizer. That is, according to Reference 3, two desulfurization methods are combined so that the first desulfurizer covers an insufficiency of a desulfurization effect of the hydrogenation desulfurizer when the fuel cell generating system is started.
According to Reference 1, the second desulfurizer used at high temperature adsorbs the sulfur compounds specifically included in LPG by use of metal oxide or the like, thereby removing the sulfur compounds from LPG. The second desulfurizer is not intended to inhibit the desulfurizing agent of the first desulfurizer from being damaged by water vapor included in the source gas. According to Reference 3, the second desulfurizer used at high temperature is intended to perform a hydrogenation desulfurization. Hydrogen is necessary for the hydrogenation desulfurization.
For example, the source gas corresponding to a city gas supplied by a gas company may include water vapor. The desulfurizing agent used at normal temperature is formed by a porous material serving as a base material. The porous material is, for example, zeolite, activated carbon, or the like. The desulfurizing agent including the porous material serving as the base material is used in a normal temperature environment, thereby adsorbing the sulfur compounds of the source gas to remove the sulfur compounds from the source gas (the desulfurizing agent used in the normal temperature environment will be hereinafter referred to as a normal-temperature desulfurizing agent). The normal-temperature desulfurizing agent including the porous material serving as the base material is inexpensive. However, for example, in a case where a source gas having a high dew point and including a large volume of water vapor is desulfurized by the normal-temperature desulfurizing agent, the normal-temperature desulfurizing agent may preferentially adsorb the water vapor to sulfur compounds of the source gas. As a result, the sulfur adsorption capacity of the normal-temperature desulfurizing agent may drastically decrease, therefore deteriorating the desulfurization effect of the normal-temperature desulfurizing agent (see FIG. 11).
A source gas having a low dew point and including a small volume of water vapor is generally supplied to the industrial world. However, for example, when a gas fitting work or a gas-pipe laying work is performed, the water vapor included in the source gas increases; therefore, the low dew point of the source gas may increase. Here, for example, the normal-temperature desulfurizing agent including the porous material such as zeolite is used at normal temperature to desulfurize the source gas having the increased dew point. In such case, the water vapor of the source gas may drastically decrease the sulfur adsorption capacity of the normal-temperature desulfurizing agent, therefore deteriorating the desulfurization effect of the normal-temperature desulfurizing agent. In addition, the normal-temperature desulfurizing agent preferentially adsorbs the water vapor to sulfur compounds of the source gas, therefore shortening a life-span of the normal-temperature desulfurizing agent. Consequently, in order to inhibit the desulfurization effect of the normal-temperature desulfurizing agent from deteriorating, a use amount of the normal-temperature desulfurizing agent needs to be increased more than necessary to desulfurize the source gas, resulting in a cost increase and an enlargement of a fuel cell system.
A need thus exists for a fuel cell system and a desulfurization unit for the same, which are not susceptible to the drawback mentioned above.